A permanent resident is allowed to live or travel outside Canada for up to 3 years out of every 5 years, with no questions asked.  In other words, you must be physically present in Canada for 2 years out of every 5 years in order to keep your permanent resident status. Please note: this rule changed in 2002.  Before that, a permanent resident could stay outside Canada only 6 months before risking losing that status.  Since 2002, this is no longer the case.

You must have a Permanent Resident Card in order to travel back to Canada.

If you are a Convention refugee, or if you need to be outside Canada for more than 3 years, your situation may be more complicated.  Talk to a lawyer.